The present invention relates to optical communications, and more particularly to optical receiving systems and methods.
In fiber optic communications networks, optical communications signals are typically transmitted in a plurality of closely-spaced communications channels within a broader communications band. For example, some optical network operators are presently using dense wavelength-division multiplexing (DWDM) to define a large number of discrete optical communications channels, such as 80 channels for example, at separate distinct wavelengths within the conventional or xe2x80x9cCxe2x80x9d-band, and to define similar numbers of optical channels in a long-wavelength or xe2x80x9cLxe2x80x9d-band, a short-wavelength or xe2x80x9cSxe2x80x9d-band, or both. As demand for data and voice communications bandwidth continues to increase, optical network operators are defining increasing numbers of discrete optical communications channels in available communications bands.
When a given optical communications channel reaches a destination point in the network at which it is to be converted into an equivalent electrical communications signal, the optical channel is typically separated (or more particularly, demultiplexed) from other channels in the band, and the optical channel is then passed to a receiver. The receiver typically includes a detector, which detects the optical signals in the channel, produces equivalent electrical signals, and amplifies the electrical signals.
However, the electrical detection process tends to introduce undesirable noise, referred to as shot noise, into the electrical signals, and similarly, the electrical amplification process tends to introduce undesirable thermal noise into the amplified electrical signals.
The relative amount of undesirable noise produced by the detection and electrical amplification processes tends to be inversely proportional to the strength of the incoming optical signals. Accordingly, one way of reducing the effects of shot and thermal noise would be to increase the strength of the incoming optical signals, by providing an optical pre-amplifier to amplify the optical signals prior to receipt by the receiver.
However, optical pre-amplifiers tend to introduce additional sources of noise. For example, rare-earth-doped optical amplifiers, such as erbium or thulium amplifiers, effectively amplify the optical signals in the desired channel by a stimulated emission process, however, a relatively small amount of undesirable amplified spontaneous emission (ASE), across a broader optical spectrum, also tends to occur. The inadvertently-produced ASE wavelengths tend to produce two main sources of noise in the optical signals, a narrow-band noise source and a wide-band noise source. The narrow-band noise source arises because ASE wavelengths in close proximity to the wavelength of the desired optical channel tend to xe2x80x9cbeatxe2x80x9d with the optical signals in the desired channel itself, producing a source of noise referred to herein as xe2x80x9csignal-spontaneousxe2x80x9d beat noise. It will be appreciated that this signal-spontaneous beat noise only occurs in a relatively narrow wavelength range surrounding the wavelength of the channel itself.
The wide-band noise source arises due to the tendency of any given ASE wavelength to xe2x80x9cbeatxe2x80x9d with any other nearby ASE wavelength, producing a source of noise referred to herein as xe2x80x9cspontaneous-spontaneousxe2x80x9d beat noise. As ASE wavelengths are produced across a broad optical spectrum, including wavelengths well outside the desired optical channel, such spontaneous-spontaneous beat noise may occur over a much broader wavelength range than signal-spontaneous beat noise. Any such spontaneous-spontaneous beat noise that occurs within the optical wavelength range to which the detector of the receiver is responsive, will produce corresponding beat noise in the equivalent electrical signals produced by the receiver.
To address this latter wide-band source of spontaneous-spontaneous beat noise, conventional wisdom dictates that an optical receiver must be provided with a filter interposed between any optical amplifier (or pre-amplifier) and the detector. The filter serves to pass only the wavelengths of the desired optical communications channel to the detector, and rejects other wavelengths, thereby rejecting most of the wavelengths at which spontaneous-spontaneous beat noise occurs (except to the minimal extent that such noise occurs within the optical channel itself).
One way of providing such a filter would be to place the optical pre-amplifier upstream of the demultiplexer used to separate the optical communications channels from one another, in which case the demultiplexer itself may serve as a filter to remove the wide-band spontaneous-spontaneous beat noise. However, the demultiplexer itself is typically a significant source of insertion loss in the optical signals due to the large number of filters it employs, and therefore, placing the demultiplexer downstream of all optical amplification would tend to weaken the optical signals, thereby partly defeating the purpose of pre-amplification, resulting in greater shot and thermal noise in the optical receiver.
Accordingly, it would be more desirable to place optical pre-amplifiers downstream of the demultiplexer, in which case the filter mandated by conventional wisdom to remove the wide-band spontaneous-spontaneous beat noise must be provided between the optical pre-amplifier and the detector. For example, such a filter may be provided as part of a receiver module including the optical pre-amplifier, followed by the filter which passes only the desired optical channel, followed by the detector.
However, providing such filters presents an expensive and inconvenient approach. It will be appreciated that a different filter is required for each one of the large number of optical communications channels in a given communications band, such as the 80 C-band channels and the 80 L-band channels presently used by some network operators, for example. This poses disadvantages for manufacturers, who must manufacture and stock a large number of different receiver filters or receiver modules containing such filters, each filter corresponding to a different respective optical communications channel.
Similarly, a number of disadvantages arise for network operators. Initial purchasing costs are increased, due to the added expense of the unique filter corresponding to each optical channel, required by each receiver or receiver module. In addition, network operators are typically required to maintain a spare part corresponding to each hardware component of the network. Thus, a network operator who uses 160 channels for example, must incur the cost of purchasing 160 different xe2x80x9csparexe2x80x9d receiver modules or receiver filters, corresponding to each of the optical channels.
Accordingly, there is a need for an improved way of receiving optical signals.
The present invention addresses the above need by providing, in accordance with a first aspect of the invention, an optical receiving method. The method involves producing amplified optical signals satisfying a filterless detection specification limit, at an optical amplifier, and receiving the amplified optical signals at an optical detector in unfiltered communication with the optical amplifier.
Thus, certain specific embodiments of the above invention may provide a number of advantages. Significantly, it has been discovered that contrary to conventional wisdom, the optical detector may be placed in unfiltered communication with the optical amplifier in many circumstances, without suffering from significant wide-band spontaneous-spontaneous beat noise. Thus, a manufacturer may produce and stock a single filterless receiver module suitable for a large number of different optical channels, rather than a different filtered receiver module for each channel. Similarly, for optical network operators, the cost of initially acquiring the receivers may be significantly reduced due to the absence of filters, and the cost of maintaining spare parts may be even more greatly reduced, as a single spare receiver module may be suitable for a large number of different optical channels, rather than merely for a single unique corresponding channel. In addition, the reduction of shot and thermal noise in the detector resulting from amplification of the optical signals is further enhanced by the unfiltered communication between the amplifier and the detector. In this regard, a filter tends to produce at least some insertion loss, and therefore, the removal of the filter results in greater net amplification of the optical signals, and a correspondingly greater decrease in shot and thermal noise in the detector.
The method may involve producing the amplified optical signals to satisfy, as the filterless detection specification limit, a pre-defined maximum ratio of spontaneous-spontaneous beat noise to signal-spontaneous beat noise. For example, this maximum ratio may be on the order of one percent. Specific examples of such filterless detection specification limits are discussed in greater detail herein.
The method may further involve selecting a physical parameter of a system including the optical amplifier, to cause the amplified optical signals to satisfy the filterless detection specification limit. For example, this may involve selecting a noise figure of the optical amplifier, or a peak noise power parameter of the optical amplifier.
In accordance with another aspect of the invention, there is provided an optical receiving system including an optical amplifier operable to produce amplified optical signals satisfying a filterless detection specification limit, and an optical detector locatable in unfiltered communication with the optical amplifier to receive the amplified optical signals therefrom. The optical detector may include a plurality of optical detectors, each locatable in unfiltered communication with a respective one of a plurality of optical amplifiers, to receive amplified optical signals therefrom.
In accordance with yet another aspect of the invention, there is provided an optical receiving system including provisions for producing amplified optical signals satisfying a filterless detection specification limit, and provisions for receiving the amplified optical signals, in unfiltered communication with the provisions for producing.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.